Dielectric barrier discharge lamp with improved color reproduction

ABSTRACT

The invention proposes a dielectric barrier discharge lamp having a phosphor mixture and improved color rendering. The phosphor mixture comprises at least the phosphor components A: cerium-activated yttrium aluminate (Y 3 Al 5 O 12 :Ce) and B: europium-activated barium magnesium aluminate (BaMgAl 10 O 17 :Eu). To further improve the color rendering and to match the emission spectrum to the spectral sensitivity of film, the phosphor mixture optionally also comprises the phosphor component C: cerium- and manganese-activated gadolinium magnesium zinc pentaborate (Gd(Zn,Mg)B 5 O 10 :(Ce, Mn)) and/or the phosphor component D: europium-activated strontium aluminate (Sr 4 Al 14 O 25 :Eu).

TECHNICAL FIELD

The invention relates to a dielectric barrier discharge lamp.

In this context, the term “dielectric barrier discharge lamp”encompasses sources of electromagnetic radiation based on dielectricbarrier gas discharges.

By definition, a dielectric barrier discharge lamp requires at least oneso-called dielectric barrier electrode. A dielectric barrier electrodeis separated from the interior of the discharge vessel or from thedischarge medium by means of a dielectric. This dielectric—whichconstitutes the actual dielectric barrier—may, for example, be designedas a dielectric layer which covers the electrode, or alternatively itmay be formed by the discharge vessel of the lamp itself, specificallyif the electrode is arranged on the outer side of the wall of thedischarge vessel. In the case of lamps in which it is defined whetherthe electrodes operate as cathodes or anodes, i.e. for operation withunipolar voltage pulses, at least the anodes are dielectricallyseparated from the discharge medium (cf. EP 0 733 266 B1).

The ionizable discharge medium usually consists of a noble gas, forexample xenon, or a gas mixture. During the gas discharge, which ispreferably operated by means of a pulsed operating method described inEP 0 733 266 B1, what are known as excimers are formed. Excimers areexcited molecules, e.g. Xe₂*, which emit electromagnetic radiation whenthey return to the generally unbonded ground state. In the case of Xe₂*,the maximum of the molecular band radiation lies at approx. 172 nm.UV/VUV radiators of this type are used in process engineering, forexample for surface cleaning, photolysis, ozone generation,metallization and UV curing. However, the present invention relates to avariant in which a phosphor layer for converting VUV radiation intoradiation with longer wavelengths, in particular visible radiation(light), is provided.

PRIOR ART

A lamp for general-purpose illumination is already known from EP 0 733266 B1, which was cited in the introduction. This lamp includes athree-band phosphor comprising the components barium magnesium aluminate(BaMgAl₁₀O₁₇:Eu), lanthanum phosphate (LaPO₄:(Tb, Ce)) and gadoliniumyttrium borate ((Gd, Y)BO₃:Eu). One drawback of this lamp is itsrelatively moderate color rendering index. However, a high colorrendering index is important for true-color reproduction of body colors.Moreover, for use in film recording technology, it is desirable for theemission spectrum of the lamp to be matched to the spectral sensitivityof the film material in order to enable corresponding objective filtersto be dispensed with as far as possible in the recording equipment. Thisproblem becomes even more acute if a plurality of different lightsources, for example for background lighting and spotlighting, withdifferent spectra are to be used. Specifically, in this case, filteringis virtually impossible.

U.S. Pat. No. 6,034,470 shows a flat dielectric barrier discharge lampwhich is likewise provided with the abovementioned three-band phosphor.This flat lamp is intended in particular for the backlighting of liquidcrystal display screens.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a dielectric barrierdischarge lamp with improved color rendering. A further aspect isimproved matching of the spectrum of the lamp to the spectralsensitivity of film material.

In a dielectric barrier discharge lamp having the features of thepreamble of claim 1, this object is achieved by the features of thecharacterizing clause of claim 1. Particularly advantageousconfigurations are to be found in the dependent claims.

The invention also claims protection for a film light having adielectric barrier discharge lamp according to the invention, inparticular also in conjunction with a dielectric barrier discharge lampaccording to the invention of flat design for diffuse illumination.

The invention proposes a dielectric barrier discharge lamp having aphosphor mixture, the phosphor mixture comprising at least the phosphorcomponents A: cerium-activated yttrium aluminate (Y₃Al₅O₁₂:Ce) and B:europium-activated barium magnesium aluminate (BaMgAl₁₀O₁₇:Eu). Thislamp is distinguished by an improved color rendering.

If the phosphor mixture does not comprise any further phosphorcomponents, it has proven expedient, in particular for a colortemperature of approx. 7100 K, if the following applies to the portionsby weight in the mixture:0.15≦A≦0.35 and 0.65≦B≦0.80 and A+B=1,preferably 0.24≦A≦0.32 and 0.68≦B≦0.76 and A+B=1.

The color rendering of the lamp can be improved further by an additionalred-emitting phosphor component C, the maximum emission of which is at awavelength of 630±15 nm and the full width at half maximum of which isgreater than 50 nm.

The phosphor component C:

-   cerium- and manganese-activated gadolinium magnesium zinc    pentaborate (Gd(Zn, Mg)B₅O₁₀:(Ce, Mn))    is particularly suitable for this purpose.

A further improvement to the color rendering of the lamp can be achievedwith an additional blue-green-emitting phosphor component D, the maximumemission of which is at a wavelength of 490±20 nm and the full width athalf maximum of which is greater than 50 nm.

The phosphor component D:

-   europium-activated strontium aluminate (Sr₄Al₁₄O₂₅:Eu)    is particularly suitable for this purpose.

Tests have shown that it is advantageous if the following applies to theproportions by weight in the mixture:0.01≦A≦0.50, 0.05≦B≦0.50, 0.05≦C≦0.70, 0≦D≦0.25 and A+B+C+D=1.

The color temperatures which are of relevance in practice, in particularbetween approx. 3200 K and 7100 K, for the lamp according to theinvention can be set within the abovementioned ranges.

In particular with a view to adapting to the spectral film sensitivityand for color temperatures of between approx. 7000 K and 5500 K, it isadvantageous if the following applies to the proportions by weight inthe mixture:0.01≦A≦0.50, 0.10≦B≦0.50, 0.05≦C≦0.50, 0≦D≦0.25 and A+B+C+D=1.

It is particularly advantageous if the following applies to theproportions by weight in the mixture:0.10≦A≦0.40, 0.15≦B≦0.40, 0.10≦C≦0.40, 0<D≦0.15 and A+B+C+D=1.

For a color temperature of approx. 6800 K, it is particularlyadvantageous if the following applies to the proportions by weight inthe mixture:0.20≦A≦0.24, 0.36≦B≦0.40, 0.29≦C≦0.33 0.07≦D≦0.11 and A+B+C+D=1.

For a color temperature of approx. 5700 K, it is particularlyadvantageous if the following applies to the proportions by weight inthe mixture:0.23≦A≦0.28, 0.30≦B≦0.39, 0.30≦C≦0.39 0.03≦D≦0.10 and A+B+C+D=1.

For a color temperature of approx. 3200 K, it is advantageous if thefollowing applies to the proportions by weight in the mixture:0.14≦A≦0.20, 0.05≦B≦0.12, 0.58≦C≦0.67 0.05≦D≦0.15 and A+B+C+D=1.

It is particularly advantageous if the following applies to theproportions by weight in the mixture:0.16≦A≦0.18, 0.07≦B≦0.09, 0.62≦C≦0.65 0.10≦D≦0.12 and A+B+C+D=1.

Specific details on the weight ratios of the individual components of adielectric barrier discharge lamp which is improved both in terms ofgood color rendering and in terms of the spectral sensitivity of filmmaterial are to be found in the description of the exemplary embodiment.

The abovementioned ranges for the proportions by weight of thecomponents of the phosphor mixture take account, inter alia, of theinaccuracies and tolerances which always occur in practice, for examplethe fact that the quantum efficiencies of different phosphor batchestypically differ slightly on account of slight fluctuations inproduction, etc. Moreover, the focal point of fine-tuning of thephosphor mixture, in particular the color temperature, may differslightly depending on the specific use. All this can be successfullycontrolled using suitable tests carried out on a small number of finelygraded mixture variations within these ranges.

In one preferred embodiment, the dielectric barrier discharge lampaccording to the invention is filled with xenon, typically with afilling pressure in the range from 50 to 200 mbar, preferably between100 and 150 mbar. As a result, the dielectric barrier dischargegenerates xenon excimer band radiation with a maximum at approx. 172 nm,which excites the phosphors. The quantum efficiency of the phosphors forthis exciting radiation should be taken into account both when selectingthe phosphor components and when selecting their ratios in the mixture,in order to ensure the highest possible light yield. A further aspect isthe maintenance of the phosphor components during long-term excitationwith this radiation. In this case, for each phosphor component, theminimum possible decrease in the intensity of the converted radiationduring the service life of the lamps is desired.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is to be explained in more detail below on the basis of anumber of exemplary embodiments. In the drawing:

FIG. 1 a shows a diagrammatic plan view of a dielectric barrierdischarge lamp according to the invention,

FIG. 1 b shows a diagrammatic side view of the lamp shown in FIG. 1 a,

FIG. 2 shows the measured, standardized spectral intensity distributionof the lamp according to the invention with a first phosphor mixture andthe color temperature 7100 K,

FIG. 3 shows the measured, standardized spectral intensity distributionof the lamp according to the invention with a different phosphor mixtureand the color temperature 6800 K,

FIG. 4 shows the measured, standardized spectral intensity distributionof the lamp according to the invention with a further phosphor mixtureand the color temperature 5700 K,

FIG. 5 shows the basic conditions of the electrode structure for a flatlamp according to the invention with a diagonal of 6.8″ which ispreferably to be operated with unipolar voltage pulses,

FIG. 6 shows the measured, standardized spectral intensity distributionof the lamp according to the invention with a further phosphor mixtureand the color temperature 3200 K.

PREFERRED EMBODIMENT OF THE INVENTION

FIGS. 1 a, 1 b respectively show a diagrammatic plan view and side viewof a dielectric barrier discharge lamp 1 according to the invention.This is a flat dielectric barrier discharge lamp for the illumination offilm recordings. Purely to simplify the illustration, a lamp with arelatively small number of electrode strips and consequently arelatively short lamp diagonal is shown. This aspect will be explainedin more detail below in connection with FIG. 5. The basic mechanical andelectrical structure of this flat lamp in any case substantiallycorresponds to that of the lamp disclosed in U.S. Pat. No. 6,034,470,which was cited above. The main difference is in the phosphor layer.Before this is dealt with in detail, the basic structure of the lamp 1according to the invention will be outlined with reference to FIGS. 1 a,1 b.

The flat lamp 1 comprises a flat discharge vessel with a rectangularbasic area and a set of electrodes arranged inside the discharge vessel.The discharge vessel for its part comprises a back plate 2, a frontplate 3 and a frame 4, in each case made from glass. Back plate 2 andfront plate 3 are in each case joined to the frame 4 in a gastightmanner by means of soldering glass 5, in such a manner that the interiorof the discharge vessel is of cuboidal form. The interior of thedischarge vessel is filled with xenon with a pressure of approx. 130mbar. The back plate 2 is larger than the front plate 3, such that thedischarge vessel has a projecting edge all the way around. Two supplyconductors 6, 7, which resemble conductor tracks, for the set ofelectrodes are applied to this edge.

The inner surface of the front plate 3 is coated with a phosphor mixture(not visible in the drawing), which converts the UV/VUV radiationgenerated by the discharge into visible white light. This phosphormixture comprises the two phosphor components Y₃Al₅O₁₂:Ce andBaMgAl₁₀O₁₇:Eu in proportions by weight of 28% and 72%, respectively.The lamp has a color temperature of 7100 K. The general color renderingindex R_(a) is 76. FIG. 2 shows the measured, standardized spectralintensity distribution (X axis: wavelength in nm; Y axis: intensitystandardized to 1) of this lamp.

FIG. 3 shows the measured, standardized spectral intensity distributionof a modified lamp comprising the four phosphor componentsA:=Y₃Al₅O₁₂:Ce, B:═BaMgAl₁₀O₁₇:Eu, C:=Gd(Zn, Mg)B₅O₁₀: (Ce, Mn) andD:=Sr₄Al₁₄O₂₅:Eu in proportions by weight of 22%, 38%, 31% and 9%,respectively. The lamp comprising this phosphor mixture is distinguishedby a general color rendering index of R_(a)=90 and a red rendering indexR_(g)=75. The color temperature is 6800 K. Furthermore, the lampspectrum is also matched to the spectral sensitivity curve of filmmaterial.

FIG. 4 shows the measured, standardized spectral intensity distributionof a variant in which only the mixing ratios of the abovementionedphosphor components A, B, C, D have been changed, specifically to 25%,33%, 37% and 5%, respectively. The spectrum is even better matched tothe spectral sensitivity curve of film material. The general colorrendering index R_(a) and the red rendering index R₉ are approx. 92 and82, respectively. The color temperature is approx. 5700 K.

A further improvement was achieved by the slightly modified mixtureA:B:C:D=0.26:0.32:0.37:0.05 (not shown).

To improve the clarity of the drawing, a representative layout of a setof electrodes for a lamp diagonal of 6.8″ for the lamp illustrated inFIGS. 1 a, 1 b is diagrammatically depicted in FIG. 5. In the case of alamp with a larger diagonal, there are no changes to the basic layout ofthe set of electrodes, but rather it is simply the case that acorrespondingly greater number of and longer electrode strips arerequired. The set of electrodes comprises a conductor track-likestructure having strip-like metallic cathodes 8 and anodes 9, 10, 11which are arranged alternately and parallel to one another on the innersurface of the back plate 2. The cathode strips 8 deliberately havespatially preferred starting points for the individual discharges formedin pulsed operation (cf. in this respect the above-cited document EP 0733 266 B1), which are realized by lug-like projections 12 facing therespective adjacent anode strip. They effect locally delimited boostingof the electric field, so that the delta-shaped individual discharges(not shown) are ignited exclusively at these locations 12. With theexception of the two outer anodes 10, 11, the other anodes 9 have adouble structure 9 a, 9 b. All the anodes 9–11 are covered with adielectric layer of soldering glass (not shown). The anodes 9–11 andcathodes 8 are each extended at one end and on the back plate 2 lead outof the interior of the discharge vessel on both sides, in such a mannerthat the associated anodic and cathodic leadthroughs are arranged onopposite sides of the back plate 2. On the edge of the back plate 2, theelectrode strips 8–11 each merge into the abovementioned cathode-side 6or anode-side 7 supply conductor. The supply conductors 6, 7 serve ascontacts for connection to preferably an electric pulsed voltage source13. For a flat lamp with, for example, a 15″ diagonal (not shown), a setof electrodes (not shown) having 12 cathode strips and 11 double anodestrips as well as two outer single anodes is provided. Each anode striphas in each case 13 projections to ignite the individual dischargesalong each of the two longitudinal sides.

FIG. 6 illustrates the measured, standardized spectral intensitydistribution (X axis: wavelength in nm; Y axis: intensity standardizedto 1) of a further variant, which is designed for a color temperature ofapprox. 3200 K. The color locus in the CIE standard color diagram hasthe coordinate x=0.421 and y=0.394. For this purpose, the proportions byweight of the four phosphor components are A:B:C:D=17.4:8.6:63.2:10.8,with A:=Y₃Al₅O₁₂:Ce, B:=BaMgAl₁₀O₁₇:Eu, C:=Gd(Zn, Mg)B₅O₁₀: (Ce, Mn) andD:=Sr₄Al₁₄O₂₅:Eu. The spectrum is well matched to the spectralsensitivity curve of the film material designed for this colortemperature and, furthermore is distinguished by the general colorrendering index R_(a)=91 and the red rendering index R₉=92.

Although the invention has been explained in detail above with referenceto the example of a flat dielectric barrier discharge lamp, it is notrestricted to this form of lamp. Rather, its advantageous effects alsoresult in the case of lamps with other shapes of vessel, for example inthe case of tubular lamps. In the latter case, the set of electrodescomprises two or more elongate electrodes which are arranged parallel tothe lamp longitudinal axis on the wall of the tubular discharge vessel.

For a film light (not shown), at least one flat dielectric barrierdischarge lamp of the type described above is mounted in a flat lightcasing, which in the simplest case is a frame in which the flat lamp(s)is(are) suspended. The lamp casing is arranged on a base, the height ofwhich can preferably be adjusted. The electrical supply unit requiredfor operation of the flat dielectric barrier discharge lamp may bearranged inside or outside the film light.

1. A dielectric barrier discharge lamp, comprising: a discharge vesselcontaining a xenon discharge medium; a set of electrodes for generatingdielectric barrier discharges in the xenon discharge medium wherebyxenon excimer radiation is emitted; and a phosphor mixture applied atleast to a portion of a wall of the discharge vessel, the phosphormixture consisting essentially of phosphor components A, B, C, and Dwherein component A is Y₃Al₅O₁₂:Ce, component B is BaMgAl₁₀O₁₇:Eu,component C is Gd(Zn,Mg)B₅O₁₀:(Ce,Mn), and component D is Sr₄Al₁₄O₂₅:Euand wherein the phosphor components have the following proportions byweight in the mixture:0.01≦A≦0.50, 0.05≦B≦0.50, 0.05≦C≦0.70, 0≦D≦0.25 and A+B+C+D=1.
 2. Thedischarge lamp as claimed in claim 1, in which the following applies tothe proportions by weight in the mixture:0.01≦A≦0.50, 0.10≦B≦0.50, 0.05≦C≦0.50, 0≦D≦0.25 and A+B+C+D=1.
 3. Thedielectric barrier discharge lamp as claimed in claim 1, in which thefollowing applies to the proportions by weight in the mixture:0.10≦A≦0.40, 0.15≦B≦0.40, 0.10≦C≦0.40, 0<D≦0.15 and A+B+C+D=1.
 4. Thedielectric barrier discharge lamp as claimed in claim 1, in which thefollowing applies to the proportions by weight in the mixture:0.20≦A≦0.24, 0.36≦B≦0.40, 0.29≦C≦0.33, 0.07≦D≦0.11 and A+B+C+D=1.
 5. Thedielectric barrier discharge lamp as claimed in claim 1, in which thefollowing applies to the proportions by weight in the mixture:0.23≦A≦0.28, 0.30≦B≦0.39, 0.30≦C≦0.39, 0.03≦D≦0.10 and A+B+C+D=1.
 6. Thedielectric barrier discharge lamp as claimed in claim 1, in which thefollowing applies to the proportions by weight in the mixture:0.14≦A≦0.20, 0.05≦B≦0.12, 0.58≦C≦0.67, 0.05≦D≦0.15 and A+B+C+D=1.